None of the official methods worked for me (not even involving horrid fixes ;)). So I ended up going the super manual route (mostly copying https://elbruno.com/2019/08/27/raspberrypi-how-to-install-dotnetcore-in-a-raspberrypi4-and-test-with-helloworld-of-course/ and https://elbruno.com/2020/01/05/raspberrypi-how-to-solve-dotnet-core-not-recognized-after-reboot/).
$ sudo apt-get install lshw$ sudo lshwdescription: Computerproduct: Raspberry Pi 4 Model B Rev 1.4serial: 10000000d5e618a2width: 64 bits; OK, it's 64bits; Get Arm64 download URL from https://dotnet.microsoft.com/download/dotnet/5.0$ mkdir temp && cd temp$ curl [URL] --output [FILENAME]$ mkdir $HOME/dotnet$ sudo tar zxf [FILENAME] -C $HOME/dotnet/$ sudo ln -s $HOME/dotnet/dotnet /usr/local/bin$ dotnet --version5.0.203; or whatever version you downloaded; Now we need to add it to PATH. Add the exports$ sudo nano ~/.bashrcexport DOTNET_ROOT=$HOME/dotnetexport PATH=$PATH:$HOME/dotnetexport PATH=$PATH:$HOME/.dotnet/tools; then Ctrl+X to save and exit. finally, load the new exports$ source ~/.bashrc